The invention relates to a method of generating an electrical output signal from an electrical input signal wherein said input signal is first clipped and then filtered into said output signal. The invention also relates to an electrical circuit for generating an output signal from an input signal comprising a clipper and a succeeding filter. Such method and circuit are known from International Patent Application WO 98/44668 A1.
In Direct Sequence Code Division Multiple Access (DS-CDMA) cellular telecommunication systems, a base station communicates with a number of mobile stations. Each link from the base station to one of the mobile stations is assigned to an exclusive channel. This enables the base station to transmit a composite signal to all of the mobile stations. Each of the receiving mobile stations then extracts its dedicated information based on the assigned channel.
The composite signal is put together of samples of the information of the channels. This signal is then input into a pulse shaping filter for creating a pulseform out of the samples. The filtered signal is then amplified. As any amplifier has a finite power peak capacity and as the composite signal may be larger than this capacity, it is possible that the composite signal is distorted.
In order to avoid or at least control such distortion, it is known to introduce a clipper before the amplifier. Such clipper decreases the amplitude of the composite signal if it is above a predefined level. Thereby, the composite signal is limited before it is amplified.
Known clippers which are located before the filter have the disadvantage that two sampled values which are both very little below the predefined level to be clipped may constitute a shaped signal after the filter which is above this level so that the amplifier exceeds its finite power peak capacity.
Clipping after the filter has the disadvantage that: out-of-band noise is generated and thereby the criteria of adjacent channel protection is violated.
To avoid these disadvantages, WO 98/44668 A1 proposes to estimate a peak-reducing waveform and to sum it with the composite signal in order to reduce the peak-to-average power ratio of the composite signal. However, this method requires complex electrical circuits with a remarkable expenditure.
It is therefore an object of the invention to provide a method and an electrical circuit which guarantee not to exceed the finite power peak capacity of the amplifier without being complex or costly.
For a method or an electrical circuit as described above, this object is solved according to the invention by said input signal being clipped dependant on said succeeding filtering.
According to the invention, the clipping is matched to the filtering, or, in other words, the clipping includes some kind of prediction with respect to the filtering. The dependency according to the invention guarantees that the clipped signal will not exceed the finite power peak capacity of the amplifier, not only after the clipper but also after the filter. Therefore, it is not possible anymore that a clipped signal becomes a shaped signal after the filter which exceeds the predefined level. This negative possibility is avoided by performing the clipping dependant on the filtering.
In a preferred embodiment of the invention, said filtering is modeled before said clipping and said input signal is clipped dependant on said modeling. The modeling of the filter is performed by introducing the coefficients of the filter into the clipper. Thereby, without requiring a complex electrical circuit, the dependency of the filtering may be realized within the clipper. Even more, the clipper according to the invention may easily be integrated into the pulse-shaping filter so that a low-cost integrated circuit may be produced.
In another preferred embodiment of the invention, said input signal is clipped such that said output signal does not exceed the predefined level. Due to the introduction of the coefficients of the filter into the clipper, it is possible to limit not only the signal produced by the clipper itself, but also the signal produced by the succeeding filter. As already mentioned, this guarantees that the succeeding amplifier does not run into its finite power peak capacity.